Much development work is underway to produce valvetrains for internal combustion engines which are more controllable than conventional cam-operated valvetrains. For operating efficiency and emission issues, it is desirable to be able to alter valve timing, opening duration and lift amount for internal combustion engines more than has been possible with cam-based systems.
Accordingly, valves operated by hydraulic actuators, under electric control, have been proposed and developed. In such systems, hydraulic fluid is applied to an actuator piston through an electrically controlled valve to move the engine valve to an open position and a conventional valve return spring returns the engine valve to the closed position when hydraulic fluid is returned, thorough the control valve, from the actuator piston.
While such systems show great promise of improvements to engine efficiencies and reduced emissions, their design and implementation involves a variety of challenges. In particular, the pump required to provide the necessary pressurized hydraulic fluid to the actuator pistons of the valve train must be reasonably efficient and yet must be capable of providing the necessary volume of relatively high pressure (approximately 1000 to 3000 PSI or more) hydraulic fluid required to operate the valve train over a wide range of engine operating speeds and conditions.
To date, it has proven difficult to provide an adequate supply of pressurized hydraulic fluid over the range of engine operating speeds from start up of the engine to high speed operating conditions.